The present invention relates generally to a method of converting color reproduction data, and particularly to a method of automatically converting three-dimensional data representing calorimetric appearance into four-dimensional CMYK (or cyan (C), magenta (M), yellow (Y), and black (K)) color reproduction data used in printing or by a color printer. This method involves maintaining color matching conditions under arbitrary K plate conditions, and in this case, involves a method of determining the arbitrary amount of the K plate formation condition automatically.
In the field of printing, a printed area is usually represented by an area modulation of a halftone percentage of CMYK. When converting arbitrary color reproduction data, such as RGB (or red (R), green (G), blue (B)), into the color reproduction data of four colors or more, including the K plate, conversion conditions, using techniques such as the separation curve or the look-up table (LUT) are determined by the user based on the purpose of the print and empirical knowledge. In this respect, determining an arbitrary K plate formation condition before color conversion, while maintaining colorimetric color reproduction data, becomes quite difficult. Generally, the mapping relationship from CMYK to three-dimensional colorimetric space, such as RGB or L*a*b and XYZ, is not one-to-one; therefore, the inverse mapping from three-dimensional space into four-dimensional CMYK is not simple, nor is the answer unique.
This method proposes a solution of treating the mapping instead as a 3-to-3 mapping of CMY to calorimetric data after restricting the K plate to a certain condition. The K plate refers to an axis in the ink color space which further includes the C,M, and Y axes. Previous techniques for color adjustment include the method of solving Neugebauer equations by successive approximation with partial differentiation in 3-to-4 conversion while holding the K plate constant, as shown in the Japanese Patent Application Opened No. 175,452/91, and the method of achieving 3-to-3 conversion with a specified K plate condition, where the K plate has been previously determined and the printed matter is subject to colorimetry (a chart), as shown in Japanese Patent Application Opened No. 158,071/91.
In the first of these conventional methods, CMYK generation (3-to-4 conversion) can only be performed where the K plate condition is specified leaving little flexibility in the printing. The second method can conceivably calculate 3-to-4 conversion as a limited solution with a previously limited K plate condition (substituted and added amounts by K plate), but the 3-to-4 conversion must be completely repeated whenever the K plate conditions are changed, thus producing processing delays for variations in the K parameter. Because the output and measurement of a similar chart are necessary, the user has a lesser degree of freedom for deciding the arbitrary K plate condition and achieving 3-to-4 conversion on that condition. In selecting the K plate amount, it is difficult to set this condition while being conscious of practical print restrictions, such as maximum ink amount.
In the printing field, a user demands arbitrary CMYK generation for every image for at least the following purposes. It is better, in printing, to reflect the general print purposes of the following typical K plate generation as a condition when deciding the K plate amount where the K plate amount refers to an ink amount corresponding to the halftone dot % of the K plate.:
1) The portion that CMY inks are printed with the fixed and equal amount is considered as gray; this portion is then substituted for the K plate as a colorimetric reproducible equivalent, securing stability and precision of color reproduction mainly of gray (and so-called achromatic plate-making is realized).
2) The total amount of ink is reduced, the balance distribution is considered, and thus, stabilization of printing is possible.
3) The maximum density realized by the three colors of CMY is increased and the range of maximum density is defined.
However, the preparation of every picture with the K plate condition that satisfies the above ideal conditions requires significant experience and skill. Setting up the K plate condition with greater flexibility, including greater degrees of freedom, and still achieving accurate conversion is even more difficult when considering both colorimetric reproduction and actual print restrictions.
Moreover, the quantitative methods involved with setting the K plate condition considering the above limitations are not well established, so this presents a difficult problem.
It is an object of the present invention to eliminate the above technical problems with the conventional methods, and to provide a color reproduction data conversion method capable of permitting the user to set the K plate condition with a higher degree of freedom (such as with respect to chroma weight, brightness weight or K plate values); to convert three-dimensional colorimetric data into four color separation data (CMYK), while maintaining the colorimetric reproduction condition of the original three-dimensional calorimetric data; to obtain reproducible four-dimensional color reproduction data suitable for printing; and to separate the data into four colors, taking into consideration any non-linearities in the conversion process.
Another object of the present invention is to provide a color reproduction data conversion method that is capable of allowing the user to optionally establish the K plate condition while maintaining the colorimetric reproduction condition; to select a parameter most suitable for every image in order to determine the K plate condition; and particularly, to automatically set up the K plate generating condition without requiring the user to have empirical knowledge about print fitness and stability, when converting three-dimensional colorimetric data, into four color separation data (CMYK), while maintaining colorimetric color matching under the arbitrary selected K plate generating condition; and to obtain the most suitable four-dimensional color reproduction data separated from four colors.
In order to achieve these objectives, the first embodiment of the invention provides for a color reproduction data conversion method for converting three-dimensional calorimetric data into four-dimensional color reproduction data containing color components of cyan (C), magenta (M), yellow (Y), and black (K) while keeping a degree of metamerism comprising the steps of:
performing an inverse conversion from three-dimensional calorimetric space to three-dimensional CMY color space as to a plurality of different K plate amounts to obtain color matching ink amounts of CMY colors being metamer with the three-dimensional colorimetric data;
obtaining an achromatic amount comprising a ratio of total ink amount of CMYK for each of the plurality of different K plate amounts to a total ink amount of CMYK in the case where the K plate amount is set to a reference amount;
obtaining an achromatic amount function for said plurality of K plate amounts in a linear portion by examining and eliminating a non-linear portion in which said achromatic amount becomes non-linear for the plurality of K plate amounts;
generating a mapping function from the three-dimensional colorimetric data and the achromatic amount to the ink amounts of CMYK in the linear portion; and obtaining a K plate control condition as a weighting coefficient which reflects a K plate control condition for the achromatic amount successively, to perform a weighting of the achromatic amount with said K plate control condition previously selected;
determining the achromatic amount from the achromatic amount function and using the selected K plate control condition as a restraint condition for the inverse conversion from the three-dimensional colorimetric data to the four-dimensional color reproduction data; and
calculating the ink amounts of CMYK from the thus determined achromatic amount and the three-dimensional colorimetric data with the use of said mapping function of the CMYK ink amount; and
printing an image based on the calculated ink amount.
Preferably, the K plate control condition would include at least a K plate substitution amount, a maximum K plate limit, a chroma weight, and a brightness weight. The K plate substitution amount refers to the sum of ink amounts substituted by the K plate in CMY ink amounts. The K plate substitution amount is usually represented as a halftone dot %.
Preferably, said inverse conversion from the three-dimensional colorimetric space to the three-dimensional CMY color space as to the plurality of the different K plate amounts is performed based on a forward conversion from a four-dimensional CMYK color space to the three-dimensional colorimetric space obtained via interpolation, calculated by making colorimetry measurements with specified color patches.
Preferably, at least one of either the CMYK ink amount mapping function or the achromatic amount function performs a K plate generation by storing as a profile of K plate generation of a previous amount, and by making successive callings.
Preferably, when a point of the color gamut outside of the print range is required for the calculation of the ink amount of CMYK, the K plate outside the color gamut is estimated as a maximum likelihood by using a parameter of the same dimension as the restraint condition used in calculating said achromatic amount function, in such a way that the K plate outside the color gamut is continued from the K plate inside the color gamut. If the thus assumed K plate is not less than 0% and not more than 100%, CMY ink amounts outside the color gamut are obtained by determining the color matching ink amounts in the assumed K plate, and if the assumed K plate is less than 0% or more than 100%, imaginary CMY ink amounts outside the gamut are estimated from a recurrence formula in a specified division space.
In a second embodiment of the invention, there is provided a color reproduction data conversion method for converting three-dimensional colorimetric data into four-dimensional color reproduction data containing color components of CMYK while keeping a degree of metamerism, comprising steps of:
setting up a restraint condition to a K plate; and
obtaining mapping of three-dimensional CMY reproduction data from the three-dimensional calorimetric data by using a forward normal lattice mapping function of three-dimensional colorimetric data from the four-dimensional color reproduction data of CMYK using successive approximation or error minimization methods;
wherein a K plate control amount is included as a parameter reflecting said restraint condition of the K plate, and said K plate control amount is established by the most suitable value demanded by previous experiment for print stability of image analytical data; and
wherein said K plate control amount comprises at least one of a K plate substitution amount and a maximum limit on K plate amount, and the analytical image data contains at least a gray histogram.
Preferably, said K plate control amount also includes at least one of either chroma weight or brightness weight.
Preferably, said image analytical data includes a basic ink gross amount of CMY, a maximum brightness, and a minimum brightness. The basic ink gross amount corresponds to a CMY ink gross amount when K=0.
Preferably, said forward normal lattice mapping function is obtained by subjecting specified color patches to colorimetry and by making the forward conversion obtained by interpolation from four-dimensional CMYK color space to three-dimensional colorimetric space.
Preferably, said K plate restraint condition is a K plate substitution ratio at each amount of the K plate. The K plate substitution ratio is a ratio of the total amount of CMYK in each amount of the K plate to the total amount of CMYK when the amount of the K plate is a reference amount. This characteristic may also be referred to as an xe2x80x9cachromatic amountxe2x80x9d The K plate substitution ratio at each amount of the K plate is obtained by making an inverse conversion from a three-dimensional colorimetric space to a three-dimensional CMY color space as to a plurality of different amounts of the K plate including the reference amount with the use of the forward normal lattice mapping function to obtain CMY ink amounts being metamer with the three-dimensional calorimetric data.
Preferably, said K plate substitution ratio is defined as the K plate substitution ratio function being a variable of the amount of the K plate in an effective reproduction area, and said K plate control amount is obtained as a weighting coefficient reflected by successive K plate substitution ratios to perform a weighting to the K plate substitution ratio.
Setting of the K plate restraint condition is performed in such a way that the most suitable K plate control amount is obtained as a parameter reflected on the K plate restraint condition for the image analytical data obtained from the three-dimensional colorimetric data, the K plate substitution ratio is weighted as the weighting coefficient with the most suitable K plate control amount thus obtained, and the K plate substitution ratio is decided from the thus set K plate control amount and the K plate substitution ratio function, and finally, the ink amounts of CMYK are calculated as four-dimensional color reproduction data.